1. Field of the Invention
This invention relates to a radiation image storage panel which is employable in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of Prior Art
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor is described, for instance, in U.S. Pat. No. 4,239,968 and is practically used. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. After the reproduction (reading) of the radiation image is completed, the remaining image is erased from the radiation image storage panel and the panel is stored for the next radiographic process, that is, the panel is generally used repeatedly after the recorded image is erased.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. Moreover, the radiation image recording and reproducing method is advantageous from the viewpoints of conservation of resources and economic efficiency because the radiation image storage panel can be repeatedly employed in the method, while the radiographic film is consumed for each radiographic process in the conventional radiography.
The radiation image storage panel employed in the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted.
As the stimulable phosphor layer, there are known not only a phosphor layer comprising a binder and a stimulable phosphor dispersed therein but also a phosphor layer composed of only an agglomerate of a stimulable phosphor (not containing a binder), which is formed by deposition process or sintering process. Also known is a radiation image storage panel having other type of a stimulable phosphor layer in which voids of a stimulable phosphor agglomerate are impregnated with a polymer material.
In each of the above phosphor layers, the stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
On the free surface (surface not facing the support) of the phosphor layer is generally provided a film (i.e., protective film) to protect the phosphor layer from chemical deterioration or physical shock.
The protective film can be formed on the phosphor layer by coating the surface of the phosphor layer with a solution of a transparent organic polymer material such as a cellulose derivative or polymethyl methacrylate, and drying the coated solution. Alternatively, the protective film can be formed on the phosphor layer by beforehand preparing a film of an organic polymer material such as polyethylene terephthalate, or a transparent glass sheet, followed by placing and fixing it onto the phosphor layer with an appropriate adhesive. Otherwise, the protective film can be also formed by depositing an inorganic material on the phosphor layer.
Among the protective films prepared by the above-described various methods, the protective film formed by coating a solution for the formation of a protective film has such advantages that the bonding strength with the phosphor layer is high and the process for the formation of the protective film is relatively simple.
In the radiation image recording and reproducing method, the radiation image storage panel is repeatedly used in a cyclic procedure comprising the steps of exposure of the panel to a radiation (i.e., recording procedure of a radiation image), irradiation of the panel with stimulating rays (i.e., read-out procedure of the recorded radiation image), and irradiation of the panel with light for erasure (i.e., erasing procedure of the remaining radiation image). Transferring of the radiation image storage panel from one step to the subsequent step is conducted using a transferring means such as a belt or a roller, and the storage panel is generally piled on other panels to be stored after one cycle is finished. When the radiation image storage panel having a protective film formed by the above-mentioned coating method is repeatedly used as described above, the sensitivity of the panel is apt to lower little by little.
Since the radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, it is desired for the radiation image storage panel employed in the method to have a high sensitivity and give an image of high quality (high sharpness, high graininess, etc.), as well as for the radiographic intensifying screen employed in the conventional radiography. Accordingly, it is very important to prevent the above-mentioned lowering (i.e., reduction) of the sensitivity of the radiation image storage panel.
As a coated protective film which is improved in the lowering of sensitivity of radiation image storage panels which is caused by the repeated use, Japanese Patent Provisional Publication No. 2(1990)-193100 (corresponding to U.S. patent application Ser. No. 07/704,738) describes a protective film of a fluorocarbon resin which is soluble in an organic solvent and is coated on a stimulable phosphor layer of a radiation image storage panel.